In order to achieve the reduction of the threshold current and the high output in the semiconductor laser, it is indispensable to reduce the propagation loss of the wave guide and the leakage current (ineffective current) which flows through the portions other than the active area layer.
In the buried heterostructure (BH) laser, the p-n junction or high resistance semiconductor layer (Fe-doped, Ti-doped or the like) has conventionally been used as the layer for blocking current and, as an example of the BH laser using the p-substrate, there is one (first conventional device) reported by Uomi et al. in the Institute of Electronics, Information and Communication Engineers, 1994 Vernal Congress, C-213 (page 4-210) and in Electronics Letters, Nov. 24th 1994, Vol. 30, No. 24, Pages 2037-2038, and Kawano et al. proposed a similar structure in Japanese Patent Application Laid-open No. 6-125131 125131/1994! (second conventional device). Further, a structure increasing the current blocking effect, which is substantially similar to the first and second conventional devices, is proposed by Terakado in Japanese Patent Application Laid-open No. 6-338654 338654/1994! and by Terakado, et al. in Electronics Letters, Dec. 7th 1995, Vol. 31, No. 25, Pages 2182-2184 (third conventional device.
The first and second conventional devices are hereinafter described with reference to FIGS. 1A to 1D, which are each a cross-sectional view for explaining the fabrication process. First, as shown in FIG. 1A, a p-InP clad layer 2, an active layer 3 and an n-InP clad layer 4 are epitaxially grown on a p-InP substrate 1 according to the metal-organic vapor phase epitaxy (MOVPE) process. Then, as shown in FIG. 1B, after a SiO.sub.2 mask 14 is formed, a mesa-stripe structure is made by etching. Next, as shown in FIG. 1C, the lateral surface of the mesa-stripe is buried with a p-InP layer 5, n-InP layer 6, p-InP layer 7 and n-InP layer 8 by growth method to form a current blocking structure. Lastly, the SiO.sub.2 mask 14 is removed and, after an n-InP layer 9 and n-InGaAsP cap layer 10 are buried flatly, as shown in FIG. 1D, a process of forming an insulation film 11 and electrodes 12, 13 is followed to form a laser structure.
Here, it is important to control the thickness of the p-InP layer 5 and the n-InP current blocking layer 6. That is, in order to avoid the interconnection between the n-InP current blocking layer 6 to the n-InP clad layer 4, when the p-InP layer 5 is grown, the thickness thereof must be controlled so that the (133) plane forming an angle of 76.+-.5 degrees relative to the (100) plane emerges. Further, the n-InP current blocking layer 6 growing on the p-InP layer 5 needs to end its growth before reaching the uppermost portion of the mesa-stripe, i.e. before coming in contact with the SiO.sub.2 mask film 14.
The above thickness control depends greatly on the width or depth of the mesa-stripe in the mesa etching process shown in FIG. 1B. Since the width and depth of the mesa-stripe are controlled by side etching the interface between the SiO.sub.2 mask film 14 and the n-InP clad layer 4, the profile of the mesa-stripe obtained by etching is greatly changed depending on the adhesion between the SiO.sub.2 film 14 and the n-InP clad layer 4, or quality of the SiO.sub.2 film 14, or concentration or temperature of the etchant. Therefore, it is difficult to fabricate the optimum structure with excellent controllability and reproducibility and with good yield.
Next, the third conventional device is described with reference to FIGS. 2A to 2D. It is basically relevant to a structure and a method which are similar to the first and second conventional devices, but as shown in FIG. 2C, the current blocking structure is different in that an InGaAsP layer 7b is present. It becomes possible to reduce the lifetime of the hole injected into the gate of a pnpn thyristor structure forming the current blocking structure by realizing the radiative recombination at this InGaAsP layer 7b so that the turn-on action of the thyristor can be suppressed. Therefore, since the leak current can be suppressed also at a high temperature, the temperature characteristic of the oscillation threshold current is excellent. However, in the third conventional device, since the mesa-stripe structure including the active layer is formed by the etching process as in the fabrication of the first and second conventional device, it is difficult to fabricate the optimum structure with good controllability, reliability, repeatability and with good yield, as in the conventional devices.
Next, a conventional semiconductor laser which is fabricated according to the MOVPE selective growth method is shown, which is excellent in controllability and reproducibility because the optical waveguide can be made without etching the semiconductor layer. FIGS. 3A to 3D are each a cross-sectional view for explaining the fabrication process of an integrated optical semiconductor device (fourth conventional device) reported by Kato et al. in the Institute of Electronics, Information and Communication Engineers, 1993 Autumn Congress, C-98 (page 4-178). First, as shown in FIG. 3A, a pair of SiO.sub.2 stripe masks 20 is formed on an n-InP substrate 21 in the direction of 011! with the separation of 1.5 to 2.0 .mu.m. Next, as shown in FIG. 3B, an n-InP clad layer 22, active layer 23 and p-InP clad layer 24 are formed on the substrate 21 at the area between a pair of the SiO.sub.2 masks 20 according to the MOVPE selective growth method. At this time, since the crystal surface of the (111)B plane is spontaneously formed as the lateral surface of the optical wave guide which is formed by the layers 22, 23, 24, the mesa stripe structure is fabricated very uniformly. Next, as shown in FIG. 3C, after a gap between the pair of SiO.sub.2 masks 20 is widened, a p-InP layer 25 and p-InGaAs cap layer 26 are epitaxially grown on the substrate 21 and the layers 22, 23, 24 according to the MOVPE selective growth method. Finally, the process of forming an insulation film 27 and a pair of electrodes 28, 29 is carried out to obtain a laser structure as shown in FIG. 3D.
As described above, in the fourth conventional device, since the optical waveguide can be formed without etching the semiconductor layer, the semiconductor laser can be fabricated with excellent controllability and reproducibility and with good yield. However, since it is difficult to introduce the current blocking structure, the threshold current rises and optical output is saturated when a large current is injected. That is, this structure has been attended with such a problem that it is difficult to achieve the reduction of the laser oscillation threshold current and the high optical output.